(a) Technical Field
The present disclosure relates generally to a method for controlling a charging voltage of an auxiliary battery, particularly a 12V auxiliary battery for a hybrid vehicle. More particularly, it relates to a method wherein the power conversion of a DC-DC converter is controlled based on factors such as the outside air temperature, the state of charge of the auxiliary battery, and the power consumption of electrical loads.
(b) Background Art
Hybrid vehicles are the vehicles of the future that employ an electric motor as an auxiliary power source as well as a gasoline engine to provide a reduction in exhaust gas and an improvement in fuel efficiency.
When the engine operates in an inefficient state, the electric motor is driven by the power of a battery to increase the efficiency of a hybrid system (load leveling). During deceleration, the battery is charged by regenerative braking, in which the kinetic energy, which would normally be dissipated as frictional heat in a brake system, is converted into electrical energy by the power generation of the motor. As such, the fuel efficiency is improved.
Hybrid vehicles are divided into soft type hybrid vehicles and hard type hybrid vehicles based on whether or not the motor is connected and driven in a power transmission system.
A motor drive system for an existing hard type hybrid vehicle is shown in FIG. 5. As shown in FIG. 5, the motor drive system includes first and second motors M1 and M2 for driving the vehicle; first and second inverters 10 and 12 for driving the first and second motors M1 and M2, respectively; a DC high voltage battery 2 for applying a DC voltage for driving the motors M1 and M2 to the first and second inverters 10 and 12; a voltage converter 14 for boosting the DC voltage from the battery 2 and supplying the boosted voltage to the first and second inverters 10 and 12, and for lowering the DC voltage from the first and second inverters 10 and 12 and supplying the lowered voltage to the battery 2; and a DC-DC converter 1 connected to the battery 2 for converting the voltage of the DC power source.
FIG. 1 shows a system for charging a 12V auxiliary battery for a hybrid vehicle and supplying power to electrical loads. As shown, a high voltage battery 2 is connected to a DC-DC converter 1 through a main switch 3. An output terminal of the DC-DC converter 1 is connected to a 12V auxiliary battery 8 and 12V electrical loads 4 (such as a variety of controllers, headlights, a water pump, a radiator cooling fan, etc.), which are driven by the power of the 12V auxiliary battery 8. As further shown, a junction box 6 is connected between the DC-DC converter 1 and the electrical loads 4 and between the 12V auxiliary battery 8 and the electrical loads 4. A wiring (parasitic) resistor 7 at the auxiliary battery side is disposed between the DC-DC converter 1 and the 12V auxiliary battery 8, and between the 12V auxiliary battery 8 and the junction box 6. A wiring (parasitic) resistor 5 is also disposed between the electrical loads 4 and the junction box 6.
In FIG. 1, VDC represents the output voltage of the DC-DC converter 1, V:J represents the voltage of the junction box 6 applied to the electrical loads 4 when the current of the electrical loads 4 is low, and VB represents the charging voltage of the 12V auxiliary battery 8. The magnitude of the voltage is in the order of VDC>V:J>>VB.
The start-up sequence of a hybrid vehicle typically includes a step in which the ignition switch (IG) is turned on by a driver, a step in which the various controllers and the main switch 3 are turned on by power from the 12V auxiliary battery 8 (the DC-DC converter 1 is not operated until the main switch 3 is turned on), and a step in which the DC-DC converter 1 operates upon completion of the start-up to charge the auxiliary battery 8 and to supply electrical power to the electrical loads 4.
In a hybrid vehicle (such as a fuel cell vehicle, a plug-in hybrid vehicle, and an electric vehicle) having the above configuration and operation, if the voltage of the DC-DC converter is maintained at a low level, the power consumption of the electrical loads is reduced, which improves the fuel efficiency. However, the amount of electrical energy charged in the auxiliary battery is reduced, which reduces the charging efficiency, thereby causing a failure during cold start-up. In particular, the DC-DC converter in the hybrid vehicle performs the functions of charging the 12V auxiliary battery and supplying power to the electrical loads of the vehicle. To improve the fuel efficiency, it is necessary to maintain the voltage supplied to the electrical loads at a lower level. It is also advantageous to increase the charging voltage within an allowable range to improve the charging efficiency of the auxiliary battery. However, if the auxiliary battery charge is low, the vehicle may not start during cold start-up due to the reduction in battery voltage, which may reduce the quality of the vehicle. In particular, as shown in FIG. 1, if the distance between the DC-DC converter 1 and the electrical loads 4 is short, and if the distance between the DC-DC converter 1 and the auxiliary battery 8 is long, then the output voltage range of the DC-DC converter 1 is increased, which further reduces the power conversion efficiency.
While the diameter of the power cable between the DC-DC converter and the auxiliary battery may be increased in an attempt to address this problem, this results in an increased weight of the vehicle, increased manufacturing costs, and a reduction in the degree of freedom in cable layout. Alternatively, while the DC-DC converter and the auxiliary battery may be disposed adjacent to each other and, at the same time, the electrical loads may be spaced away from each other in an attempt to address this problem, this also reduces the degree of freedom in vehicle design and, further, reduces the quality of the vehicle due to various limitations such as cooling of the DC-DC converter and the like.
As mentioned above, the DC-DC converter 1 operates upon completion of the start-up to charge the auxiliary battery 8 and to supply electrical power to the electrical loads 4. At this time, if the voltage of the auxiliary battery is less than 9V, the main switch 3 generally may not be turned on (however, it is noted that various controllers normally operate at a voltage of more than 6V), and thus the DC-DC converter is not connected to the high voltage battery 2. As a result, the auxiliary battery is not charged, and the voltage of the auxiliary battery may be further reduced during the cold start-up.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.